AdWords scripts simplifies one-off changes that touch multiple account entities, provides a way to interoperate with external data and open possibilities for automated management of campaigns and accounts.
AdWords scripts are mainly used for:
creating custom advanced reports that are not available by the way of the web interface.
using external data (products, weather, prices, etc.) for managing campaigns and bids.
to make change for all campaigns or ad groups with a single click
The script manager and editor tools can be accessed in the Campaigns tab, under Bulk operations in the left navigation bar.
The AdWords script editor:
A video introduction to AdWords scripts: